IDA Scripting Library
IDA Plugins & IDAPython Scripting Library.
The library is in early development, so APIs are expected to change.
For documentation, see sark.rtfd.org.
pip install sark
Or for latest version:
pip install -U git+https://github.com/tmr232/Sark.git#egg=Sark
For more info see here.
Other Useful Plugins
IDA IPython - Run IPython inside IDA.
Download the file for your platform. If you're not sure which to choose, learn more about installing packages.